1. Field of Invention
The invention relates to a biometrics method based on a thermal image of a finger, and more particularly to a method of verifying a user's identity according to a finger-print image and a finger-vein image. The fingerprint sensing chip adapted to the invention has been disclosed in the commonly assigned U.S. Patent Publication No. US2004/0208345A1.
2. Related Art
The conventional biometrics verification may be performed based on a fingerprint of a finger of a user. The fingerprint recognition has the advantages of the high precision and the convenience and has the international standard of algorithm. However, some users having special works may have no fingerprint. For example, the finger of the cement worker or the chemical worker is eroded by cement or chemicals for a long time so that no obvious fingerprint exists on the finger. Any fingerprint sensor, such as the optical-type sensor, the pressure-type sensor, the capacitive sensor or the electric field type sensor, cannot acquire the fingerprint of this kind of user. Some techniques have declared that the fingerprint on the derma layer under the skin can be sensed. However, contamination, oil and any other substance are usually attached to the finger and thus interfere with the detection of the derma layer. If the finger is wetted, the conventional fingerprint sensor device cannot sense the good finger-print image. Thus, the spirit of the invention is to provide a biometrics method capable of overcoming the problems induced when no fingerprint exists on the finger and when the finger is wetted.
To solve the above-mentioned problems, the present inventor has disclosed a thermoelectric sensor for fingerprint thermal imaging disclosed in US2004/0208345A1. An array element is manufactured according to the thermoelectric sensing principle so that the finger ridges and valleys can be sensed. The sensing member works based on the principle of sensing the contacted body heat conduction (temperature difference) or the principle of sensing thermal infrared emitting from the finger to the sensing members in a small gap. Different substances, such as water and the skin (the wetted finger) or air and the skin (the dry finger) have different thermoconductive properties and different infrared material properties. Thus, different images with different contrasts can be obtained so that the fingerprint recognition can be performed. The invention further provides the method of sensing the veins of the finger based on the sensing principle of the sensor so that the user without the obvious fingerprint pattern on the finger can be verified and the true finger and the fake finger can be distinguished.
FIG. 6 is a schematic illustration showing a thermoelectric sensor for fingerprint thermal imaging disclosed in US2004/0208345A1. As shown in FIG. 6, each sensing member 10′ is formed by integrated circuit manufacturing processes, especially the CMOS manufacturing processes. The basic structure of the sensing member 10′ includes a silicon substrate 100, a local oxidation of silicon (LOCOS) layer 101, a thermopile composed of at least one thermocouple 102 or many thermocouples connected in series, and a heat pipe 400. The thermopile has a hot junction 200 disposed at a central portion of the LOCOS layer 101, and a cold junction 300 disposed on a thin oxide layer (not shown) around the LOCOS layer 101. The structure of the heat pipe 400 includes at least one metal interconnect layer and at least one via hole metal plug. The heat pipe 400 is formed between the central portion of the LOCOS layer 101 and a passivation layer 106 on the outermost surface.
The fingerprint includes a fingerprint ridge 20 and a fingerprint valley 21. When the fingerprint ridge 20 contacts or approaches the sensing member 10′, heat or infrared radiation is generated between the fingerprint ridge 20 and the sensing member 10′, and is transferred through the solid heat conducting mechanism in directions as indicated by the two arrows. Most heat can be conducted to the hot junction 200 of the thermopile on the LOCOS layer 101 through the heat pipe 400, and then transferred in various directions through the hot junction 200. Therefore, a temperature difference ΔT is generated between the hot junction 200 and the cold junction 300. The sensing member 10′ senses a voltage signal through the temperature difference ΔT in order to judge whether it contacts with the fingerprint ridge 20 or not. The voltage signal generated by the sensing member 10′ may be represented by the following equation:V=NαΔT  (1),wherein N is the number of the thermocouples connected in series, and α is a Seebeck coefficient (V/° C.) of a single thermocouple.
It is to be noted that the heat pipe 400 can enhance the temperature difference in order to produce the maximum temperature difference between the hot junction 200 and the cold junction 300 of the thermocouple 102. According to this property, the finger-print image and the finger-vein image can be sensed so that the spirit of US2004/0208345A1 can be extended.